1. Field of the Invention
The present invention relates to a method for manufacturing a reduced metal by thermally reducing a metal oxide including a carbonaceous reductant with a moving hearth reducing furnace.
2. Description of the Related Art
In a method for manufacturing reduced iron (a reduced metal) with a rotary hearth reducing furnace (a moving hearth reducing furnace), coal (a carbonaceous reductant) is added to iron ore including iron oxide (a metal oxide), the mixture is charged on a hearth that moves in the reducing furnace, and the mixture is heated at a high temperature in the reducing furnace to reduce the iron oxide (the metal oxide) in the iron ore by the coal (the carbonaceous reductant), thus forming reduced iron (the reduced metal).
In the reducing furnace, a ring-shaped hearth is rotatably supported in the circumferential direction in a reducing furnace main unit having a doughnut shape. The reducing furnace includes a material-feeding unit for feeding the iron ore including the coal on a predetermined position of the hearth in the reducing furnace, and a product-discharging unit for discharging the reduced iron that is reduced in the furnace to the outside. Furthermore, a space that forms a high temperature atmosphere is formed on the hearth in the reducing furnace. The reducing furnace further includes primary burners for supplying fuel and primary combustion air to the space, secondary combustion burners for supplying secondary combustion air, and an exhaust gas duct for discharging exhaust gas generated by combustion in the furnace.
As described above, while the material-feeding unit feeds the iron ore including the coal on the hearth, the hearth rotates at a predetermined rate and the primary burners heat the space to form the high temperature atmosphere. Thus, the iron oxide in the iron ore on the hearth is reduced by the coal to form reduced iron while moving in the high temperature atmosphere in the furnace. The reduced iron is discharged from the product-discharging unit to the outside of the furnace.
The iron ore including the coal, which is heated while moving in the high temperature atmosphere in the furnace, generates a volatile matter generated from the coal, and CO gas generated by reductive reaction between the coal and the iron oxide in the iron ore. The volatile matter and the CO gas are hereinafter generically called combustible gas. In order to use the combustible gas efficiently, the secondary combustion burners supply the secondary combustion air, thereby properly burning the combustible gas in the furnace. Thus, the consumption of the fuel supplied from the primary burners can be reduced.
Furthermore, in a proposed method, preheated air or oxygen-enriched air is used for the primary combustion air and the secondary combustion air. The effect of using the oxygen-enriched air as the primary combustion air and the secondary combustion air is achieved by decreasing the content of nitrogen in the combustion air. That is, decreasing the content of nitrogen in the combustion air decreases the heat value for heating the nitrogen that barely contributes to heat the materials. Accordingly, this method decreases the consumption of the fuel supplied from the primary burners. Furthermore, since the amount of the exhaust gas discharged from the reducing furnace is decreased, the processing equipment of the exhaust gas can be reduced in size, thereby reducing the operating cost and the equipment cost.
The decrease of the nitrogen content in the combustion air increases the temperature of burned gas. In this case, the generation of nitrogen oxides (NOx) increases, thereby increasing the NOx content in the exhaust gas in the reducing furnace. Accordingly, NOx removal equipment is required. Furthermore, for example, the equipment must be large-scaled and the amount of the equipment must be increased in order to take measures to reduce the NOx. Unfortunately, the increases of the area for installing the equipment, the operating cost, and the equipment cost cancel out the advantage due to the decrease of the fuel consumption. Any solution for this problem is not proposed hitherto; therefore, the practical application of the method is inhibited.